Aseptic packaging technology has for a long time been used for packaging foodstuffs and the like, especially products sensitive to bacteria and storage, in order to give the product an extended shelf life so that it can be retained for longer periods of time from the day of packaging without risk of being spoiled or deteriorating. The technology is well known to an artisan and can for example start out from the fact that the product and the packaging material are each subjected to a sterilising treatment for the purpose of neutralising harmful micro-organisms occurring in the product of the packaging material, and that the treated product is thereafter enclosed in the sterilised packaging material under sterile conditions in order to avoid a bacterial reinfection of the sterilised product.
Aseptic packagings for milk, juice and similar liquid foods are now most frequently produced with the aid of packaging machines of the type that, from a strip or sheet of plastic coated paper or cardboard material, forms, fills and closes packages under aseptic conditions. From, for example, in a known packaging machine that produces aseptic packagings for milk, the strip is led through a bath containing 10-35% by weight of hydrogen peroxide within a chamber essentially completely screened from the environment. After the passage through the bath the strip is pinched between press rollers in order to remove the surplus sterilising agent from the strip and return it to the bath.
Thereafter the strip, without coming into contact with the machine environment is led into the forming and filling chamber of the machine, which chamber is likewise essentially completely screened from the environment. In the forming and filling chamber, the packaging material is heated by means for sterile hot air in order to vaporise and drive away the residue of accompanying sterilising agent from the packaging material, whereby the strip is shaped into a tube by sealing the two longitudinal edges to each other in a longitudinal overlap joint. The tube is filled with the appropriate previously sterilised contents, heat treated milk or the like, which is fed to the tube through a filler pipe opening into the tube, and separated into closed, filled packagings through repeated transverse sealings of the tube across the longitudinal axis of the tube.
The packagings are separated from each other through cuts in the transverse sealings and are subsequently given the desired geometric final shape, usually of parallelepipedic type, before out feed of the finished aseptic packagings from the machine. During the whole process an overpressure of sterile hot air is maintained in the shaping and filling chamber in order to prevent unsterile environmental air from penetrating and reinfecting the sterilised contents and the packaging material.
A precondition for achieving good sterilisation of the packaging material in the above-described known method is that the whole strip, after passing through the sterilising bath, is covered by a coherent film of sterilising agent in order to ensure that all parts of the packaging material are effectively sterilised. The film should in addition preferably be thin and of even thickness in order to facilitate and make more effective the subsequent vaporisation of the sterilising agent in the shaping and filling chamber. These two conditions have been shown difficult to fulfil in practice and it not infrequently happens that the sterilising agent exhibits an irregular film thickness over the strip surface, which not only leads to an uneven and unpredictable sterilisation effect but also makes the vaporisation more difficult.
Another disadvantage with the above described and other known methods, which employ a bath of sterilising agent through which the strip is trained, is the edge suction phenomenon which entails that the exposed fibrous material in the longitudinal cut edges have a tendency to absorb the liquid sterilising agent which is retained in the fibrous layer of the strip. Since the inward facing cut edges in the finished packagings are always well protected no risk occurs that accompanying sterilising agent should come in contact with and affect the contents of the packaging, but on the contrary the risk is great that the liquid absorbed will at least locally cause deterioration in the rigidity and stability of form of the packaging at the same time as it of course entails an unnecessary loss of sterilising agent.
Another problem associated with this type of sterilising method occurs when the sterilising method is applied to packaging materials including pre-applied opening devices. The pre-applied openings act as shovels or bailers transporting considerable amounts of sterilising agent from the bath. Due to the conventional shape of the opening devices the above-mentioned press rollers are not able to prevent this phenomenon from occurring.
EP 484730 A1 discloses a method of sterilising a packaging material by means of a sterilising agent in a liquid form. The packaging material is discharged in order to eliminate the electrostatic surface charges on the packaging material. Thereafter the packaging material is wetted with the sterilising agent, which is applied in as a finely distributed, electrostatically charged, mist to those areas of the packaging material that are to be sterilised. The packaging material with the sterilising agent, which has coalesced to a thin film, is finally heated to vaporize the sterilising agent from the ready sterilised packaging material. EP 484730 A1 is presently considered closest prior art.
However, this method also suffers some drawbacks; it is hard to properly discharge the packaging material since the training of the packaging material around bending rolls, forming rolls, etc induces quite a lot electrostatic charge of the packaging material. Furthermore, the mist of sterilising agent does not result in an even film on the packaging material.